This invention pertains generally to semiactive guidance control systems for guided missiles and particularly to a system of such kind in which the operating frequency of a reference oscillator in a guided missile during flight is controlled to maintain coherence between such operating frequency and the frequency of a control radar.
It is known in the art, as shown in the now pending U.S. patent application, Ser. No. 579,281, entitled "Adaptive Semiactive Missile Guidance System and Elements Therefor," and assigned to the same assignee as this application, that an electronically tunable arrangement may be used in a guided missile to maintain coherence between the operating frequency of a reference oscillator in such missile during flight and the frequency of a control radar. As described in detail in that application, the electronically tunable arrangement includes an Yttrium-Iron-Garnet (or YIG) filter as the frequency determining element for the reference oscillator. Briefly, in the referenced arrangement, portions of the output signal from a voltage-controlled oscillator (the first local oscillator) are passed through a YIG filter and a passive phase shifter to the input terminals of a phase detector. The signal out of such detector, then, is indicative of the difference between the frequency of the output signal of the voltage controlled oscillator (the first local oscillator signal) and the resonant frequency of the YIG filter. The signal out of the phase detector then, after appropriate shaping, is applied to the voltage controlled oscillator to force the frequency of the output signal from that element into coincidence with the resonant frequency of the YIG filter.
In order to tune the voltage controlled oscillator to the proper frequency with respect to the frequency of the radar signal from the control radar when the guided missile is launched, the resonant frequency of the YIG filter is changed in a programmed manner until an output signal is produced by the rear receiver. Unfortunately, however, when more than one control radar is being operated in the vicinity (as when a guided missile is launched from one aircraft in a formation) of the launching point of a guided missile, the programmed change in the resonant frequency of the YIG filter may cause an output signal to be produced when the radar signal in the rear receiver is from the wrong control radar. The guided missile, then, either is directed toward a target selected by the wrong radar or control of such missile is lost.